The present invention relates to a process and plant for the continuous manufacturing of at least a layer of latex foam of unlimited length comprising a plurality of recesses in all its extent derived from respective protuberances arranged on a laying surface of the latex foam continuously advancing.
The layer is divided into a plurality of blocks of latex foam by transverse cuts in order to manufacture rest items such as pillows and particularly mattresses wherein the aforementioned latex foam blocks are between and 20 cm thick and more. More specifically the invention relates to the aforementioned processes where processing steps and plant devices capable not only of forming a latex foam layer having a uniform density but also of affecting the characteristics of the end product with greater stability and lower costs than the known art are achieved.
Continuous processes for the continuous manufacturing of latex foam layers provided with recesses are known.
To this end Patent EP 0 358914 describes a continuous process wherein a laying surface of the latex foam consisting of a plurality of metal plates provided with aluminium protuberances is continuously advanced according to an annular path between two pulleys, a driving pulley and a driven pulley.
The plates move along an upper branch of the annular path between a first laying station and a extracting station of the continuous layer passing through adapted heating devices, for the known gelating and curing steps, placed between the two aforementioned stations.
All through its cellular body, the layer receives heat necessary to its stability either through said adapted heating devices or through heat coming from the metal plates and the protuberances heating.
It should be noted that the state of the art described in this Patent mentions some continuous processes wherein the latex foam deposits on a protuberance-free metal band to form items of limited thickness, up to 5 cm.
This process, as described in the aforementioned Patent, is unsuitable for forming latex foam layers having great thickness, for mattresses formation for example, as the absence of the protuberances prevents enough heat transfer through the whole cellular body thus preventing gelating and curing of the latex foam layer.
Thus, as stated in Patent EP 0 358914 and as known to those skilled in the art, continuous processes with protuberance-free laying surfaces for the foam do not allow the manufacturing of mattresses having a thickness between 14 and 18 cm and recesses having a depth of 11 cm, for example.
These known continuous protuberance-free processes and plant are usually used to form latex foam layers having a small thickness and usually employed in manufacturing carpets, sanitary packs or coatings.
Other processes which make use of a laying surface and metal protuberances adapted to continuous formation of unlimited length latex foam layers with recesses having a high thickness between 10 and 20 cm and more are already known.
To this end reference is made to Patent EP 0 955 144 wherein the laying surface consists of a plurality of carriages each provided with a laying surface and a plurality of protuberances.
The carriages move along an annular path between two pulleys.
The continuous processes described in Patents EP 0 358914 and EP 0 955 144 are suited to manufacture latex foam blocks provided with recesses and having a high thickness and their characteristics and performances prove to be satisfactory.
For a better understanding of the invention to be described afterwards, it is worth noticing how the various mattress manufacturers compete in the attempt to achieve more and more significant results in order to account for improvements of specific characteristics of the product. In this perspective the different steps of the process have been gone through in search of further conditions which, thoroughly investigated, could lead to improvements unexpected so far.
It should be considered that the continuous processes, which will be referred to later, make use of latex foam the laying of which on the laying surface requires great care as the latex foam is so unstable that might cause collapse in the cells of the material.
In order to face up with such a risk, the gelating process is performed which prevents the latex foam to collapse after its deposit. However it must be clear that the latex foam collapsing phenomenon affects the whole manufacturing cycle.
Consequently though the gelating step and the successive curing step are accurate, in the end product it has been observed a variation in density of the foam with values in the recesses' area lower than in the area above the recesses.
Such a variation in density has been ascribed to the maximum instability condition of the foam that is when the gelating step has not taken place yet.
Before the gelating step, the latex foam tends to collapse deeper in the area among the protuberances of the laying surface than in the area above the protuberances.
Special attention has been drawn to the presence of closed air bubbles close to the ends of recesses. This is not a minor problem as it affects the foam layer in all its length thus creating, for lengths equal to the layer longitudinal size, for example for lengths in the order of hundreds of meters and more, innumerable bubbles which cause a lesser density in the area of their localization than in the areas where the bubbles are absent.
It has been thought that such air bubbles were due to the maximum instability of the foam so once again during the foam laying before the gelating step.
In order to ensure a uniform density of the foam all through the definitive product, it was then decided to make use of means capable of giving support to the latex foam among the protuberances and of means capable of draining the air bubbles off otherwise present at the protuberance ends.
Thus it has been perceived possible to create means capable of applying supporting forces substantially similar to those exerted by the protuberances towards the foam and adapted to offer escape routes to air bubbles thanks to their transpiration characteristics.